Doping-Induced Structural Phase Transition in Na_1.6−xCo[Fe(CN)₆]_0.902.9H₂O

Abstract

Lattice structure of hole-doped Na_1.6−xCo[Fe(CN)₆]_0.902.9H₂O was systematically investigated against doping level (x) and temperature (T) by means of synchrotron-radiation powder diffraction. We found that the hole-doping procedure on the Co site induces a rhombohedral–cubic structural phase transition. The rhombohedral phase (cubic phase) is observed in x≤0.02 and 100≤T≤300 K (x≥0.14 and 100≤T≤300 K), while two lattice structures coexist in the intermediate-x region (0.02≤x≤0.14). We interpreted the two-phase region of the x–T phase diagram in terms of intrinsic phase separation into large-x (Na⁺-poor) and small-x (Na⁺-rich) regions, because the Na⁺ ions can migrate within the crystal to minimize the total free energy.